1. Field of the Invention
The invention relates generally slickline cables used in oilfield operations.
2. Background Art
Fiber optic components in wireline or permanent monitoring cables have a great potential for data transfer applications. Unfortunately, this potential is offset by several weaknesses that make them vulnerable to damage in oilfield operations. For example, exposure to hydrogen at high temperatures results in darkening of the optical fiber which leads to a reduction in data carrying capacity. Evaporation of volatile organic compounds (VOCs) in coatings or other polymeric protective layers on the optical fibers releases hydrogen, which can attack and darken the fiber. Hydrolytic attack against glass in the presence of water is yet another source of damage.
Furthermore, linear stretch of the fiber is limited when compared to the other cable components. This requires additional fiber length in the optical fiber components, which complicates the manufacturing process. A lack of transverse toughness of the fiber component construction can result in potential point loading and micro-bending issues. These can lead to mechanical failure of the fiber and/or signal attenuation.
FIG. 1A shows a typical slickline cable. As shown, a slickline cable 100 consists of an optical fiber 110 contained in an inner steel tube 120, which is coated with a polymer (which may be a continuous or long fiber-reinforced) composite 130. An insulation layer 140, which may comprise thermoset resin such as epoxy, or other thermoplastic, is then added over the polymer composite 130 to complete the slickline core. Finally, an outer steel tube 150 is drawn over the slickline core to complete the slickline cable 100.
Several problems have been encountered with this design. The polymer composite 130 in the slickline core may become oval during manufacture. The inner steel tube 120 can move off center within the polymer composite 130. When the polymer composite 130 between the inner steel tube 120 and outer steel tube 150 has insufficient thickness (e.g., due to shrinkage or other factors), the polymer composite 130 and the outer steel tube 150 can separate from each other when the cable is flexed or spooled over sheaves.
In addition, different materials used in the slickline cable may have different coefficients of thermal expansion, which may cause some problems. For example, during manufacture, the polymer composite 130 and the inner steel tube 120 tend to swell. As the polymer composite 130 cures and cools, it tends to contract and pull away from the inner steel tube 120. Additionally, the inner steel tube 120 shrinks more in the longitudinal direction than does the polymer composite 130. During pultrusion and curing of the composite, the optical fiber is subjected to curing temperatures between 400 to 500° F. for a short period of time, which can damage the fiber's polymer coating. Also, the fiber finish of the composite may interfere with the epoxy curing.
One approach to an improved slickline cable is to coat the optic fiber with a resin jacket to form a more rugged fiber optic. FIG. 1B shows one such fiber optic having a continuous or long-fiber-reinforced epoxy thermoset resin jacket 115 over a commercially obtained optical fiber 110.
Although a fiber optic as shown in FIG. 1B is more robust, the processes of covering the fiber optics with the composite resin jackets may lead to some problems. For example, high loss of optical fiber can occur due to point loading in the pultrusion process. Shrinkage that occurs as the epoxy cures can impinge on the optical fiber and create signal attenuation problems. The need to handle the optical fibers carefully in order to reduce the likelihood of point loading and overpull in the pultrusion process makes manufacturing difficult and time-consuming. The high incidence of signal attenuation encountered with these fiber optic components is unacceptable for use in oilfield distributed temperature system measurements and in applications requiring long-length telemetry.